Quality of welded zones is of crucial importance for welded steel pipes, and, in manufacturing processes, on-line flaw detection is generally conducted on welded zones through ultrasonic angle-beam flaw detection. This technique involves causing an ultrasonic wave to be obliquely incident on the surface of the work to be inspected and detecting inner/outer surface defects and subsurface defects of the work on the basis of the reflected waves from the defects. Typically, for example, a reflection method using an ultrasound beam having a 45° refractive angle at 5 MHz is applied to electric resistance welded steel pipes to detect millimeter-order defects, e.g., poor penetration, burn-through, and cracks caused by inclusions.
Recently, the quality requirements for welded steel pipes have become stringent, and detection of flaws smaller than those conventionally detected is required. Examples thereof are cold junction flaws and minute penetrators in electric resistance welded steel pipes and blowholes in laser welded pipes. These flaws range from several tens to several hundred micrometers in size, which are extremely small. These flaws can occur at any locations along a weld line from the inner surface to the outer surface, and the point incidence of the ultrasound beam may differ from its point of return depending on the positions of the flaws. Due to these factors, conventionally practiced ultrasonic flaw detection techniques frequently fail to detect such flaws, and a more accurate flaw detection technique that can be used in quality control of steel pipe welded zones has been desired.
The following related arts have been disclosed as a method for detecting minute flaws for use in quality control of welded steel pipes. Japanese Unexamined Patent Application Publication No. 60-205356 describes use of a point focus-type probe having a frequency of 8 MHz or higher in angle beam testing to improve the detectability for penetrators. Japanese Unexamined Patent Application Publication No. 11-183446 describes detection of blowholes by sector-scanning a welded zone from the inner surface side to the outer surface side with an array probe that forms a focus beam to improve the detectability.
Japanese Unexamined Patent Application Publication No. 61-111461 describes detection of cold joint flaws, which are mixed minute FeO grains several micrometers or smaller in size taking form of clusters, by causing an ultrasonic wave having a frequency of 25 MHz to 500 MHz to be incident on the welded zone from the pipe outer surface-side at an angle of incidence of 0° to 20°. Japanese Unexamined Patent Application Publication No. 7-35729 describes use of a plurality of 20-80 MHz point focus-type probes arranged so that their focusing positions are arranged at a pitch of 3 mm or less from directly above the seam to detect blowholes 0.1 mm or more in size.
It should be noted here that in the Disclosure of the Invention, Japanese Unexamined Patent Application Publication No. 4-274756 and “Ultrasonic Flaw Detection Series (II)—Ultrasonic flaw detection of welded steel pipes” edited by the Iron and Steel Institute of Japan, 1988, pp. 28-31 are cited.
However, the disclosed techniques described above still have the following problems. First, for the technique disclosed in Japanese Unexamined Patent Application Publication No. 60-205356, since the beam width of the focused ultrasonic wave is small, a large number of channels are needed to completely detect flaws in all parts of the welded zone in the depth direction (thickness direction of the steel pipes) and this increases the equipment cost. Moreover, the centering control or the like required in the event of pipe size change is very cumbersome. Furthermore, when the flaw shape is not blowhole-like but is planar such as in the case of penetrators and cold junctions and when such a flaw is located inside the wall, the reflected wave travels in a direction different from the direction of incidence and this renders it difficult to detect such a flaw.
According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 11-183446, only one array probe is needed and the setting required in the event of size change can be conducted electronically. Thus, although this technique does not face the first problem of Japanese Unexamined. Patent Application Publication No. 60-205356 but the second problem remains unresolved.
When the flaw shape is planar as described above, for example in the case of electric resistance welded steel pipes, the seam zone is upset and thus the width of the flaw as viewed from directly above the seam is as small as 100 μm or less. Thus, even with the techniques described in Japanese Unexamined Patent. Application Publication No. 61-111461 and Japanese Unexamined Patent Application Publication No. 7-35729, the reflected waves from the flaws are actually very weak and difficult to detect in many cases. Moreover, since about 1 to 2 mm of the portion near the surface echo forms a dead zone due to reverberation of the surface echo, there is another problem that flaws located near the outer surface cannot be detected.
As discussed above, according to the techniques for detecting minute flaws about several hundred micrometers or smaller in size that occur in the welded zones of the welded pipes in the pipe axial direction, the detection performance is not sufficient and these techniques have difficulty meeting the stringent quality control requirements of recent years. Development of technology that resolves these problems has been desired.
The present invention has been made under the above-described circumstances and aims to assuredly conduct quality control of electric resistance welded steel pipes that require stringent quality control.